Exoplanets evolve in the radiation environment created by their host stars. Of particular interest is the UV and X-ray emission produced in the chromospheres and coronae of cooler stars such as M dwarfs. I will summarize the results of our HST MUSCLES Treasury Survey, consisting of complete UV spectra of 7 M-type dwarf stars and 4 K-type dwarf stars, together with coordinated X-ray and optical observations of these stars. The spectral energy distributions of the M stars are very different from G-type stars like the Sun with much stronger far-UV, extreme-UV, and X-ray emission compared to the optical emission. The far-UV emission, in particular the very bright Lyman-alpha line, photodissociates H20, CO2, and CH4 in the outer atmospheres of exoplanets, and the extreme-UV and X-ray emission is the energy source for exoplanet mass loss. I will describe how we correct the observed Lyman-alpha emission from interstellar absorption and present our new model of the chromosphere and corona of the M1.5 V host star GJ 832.
